S-wave Reflection Research Articles

Preliminary Magnetotelluric Modeling in the Nikko Volcanic Area. Potential Break of Fluid Trap by Volcanic Intrusion.

We collected magnetotelluric data in the Nikko volcanic area in the period range from 0.01 to 10 sec. Impedance data were decomposed with the regional strike direction N0°E and were analized by two-dimensional inversions including static shifts as model parameters. The major model features are: (1) southern conductor at 8 km depth which is consistent with the seismic S-wave reflectors, (2) northern conductor located at 5 km depth, consistent with the seismic attenuation zones, and (3) a resistitive gap in between, consistent with the magnetic body inferred from aeromagnetic data. The two conductors probably imply trapped free water at around 400°C. The resistor in between implies the volcanic body, which intruded the crust and may have broken the regime of trapped free water.

Vertical seismic profile results from the Kola Superdeep Borehole, Russia

Multi-offset vertical seismic profiles (VSPs) from the Kola Superdeep Borehole (SG-3), as part of a larger seismic study of the Kola region conducted during the spring of 1992, sample the dipping Pechenga complex from 2175 m to 6000 m and contribute to the understanding of reflectivity in crystalline and Precambrian environments. From the surface to 6000 m, the SG-3 borehole penetrates interlayered Proterozoic metavolcanic and metasedimentary units and a mylonitic shear zone ranging from greenschist to amphibolite metamorphic grade, respectively. The Kola VSPs display a 6% velocity decrease which coincides to a mylonitic shear zone located between 4500 m and 5100 m within the SG-3 borehole. Seismic interfaces are identified by mode-converted energy (PS, and SP transmissions and reflections) in addition to primary seismic phases. The VSP shear wave energy is generated at or near the source by vertical vibrators. P-wave and S-wave reflections are generally detected from the same reflecting horizons, but increases in relative S-wave and SP reflection amplitudes originate at 1900 m, 3800 m, 4500 m, and 5100 m depths. These depths coincide with zones of elevated V p V s and may support the presence of free pore fluid which is reported from initial drilling. For the Proterozoic lithologies sampled by the VSP, reflection events result from five mylonitic shear zones and three lithologic contrasts.

Nature of seismic reflections and velocities from VSP-experiments and borehole measurements at the KTB deep drilling site in southeast Germany

The 1989 VSP program at the KTB pilot hole was complemented in 1992 by a standard VSP in the KTB super-deep borehole from 3000 to 6013 m. P-wave velocities oscillate around 5.8 km/s in the upper 3150 m in accordance with sonic log velocities and correlate with paragneisses which prevail in this depth range. At about 3150 m depth, velocities increase to 6.4 km/s correlating with metabasites which dominate in the depth range 3150 to 7500 m. Laboratory measurements and petrophysical modelling provide evidence that the intrinsic velocities were reduced by 5–10% by fracture density and porosity at all depth ranges. Subvertical dip (50–70°) in structures and textures prevail, causing about 10% S-wave anisotropy, indicated by direct observations of S-wave splitting. Pronounced P-wave reflections, accompanied by P- to S-wave conversions and a lack of S-wave reflections, occur in the lower depth range only (3000–6000 m) and correlate with fluid-filled fracture systems. Lithological contrasts (gneiss-amphibolite) play a minor role in generating reflections. The most prominent reflecting elements known from surface profiling and 3D-surveys (i.e. the ‘Franconian Lineament’ reflector at about 7 km, and P-wave reflections at 8.3 km with notably absent associated S-wave reflections) coincide with pronounced anomalies observed in the logging data indicating the presence of major fracture zones.

Reflectivity characteristics of the crust from modelling refraction/wide-angle and normal-incidence reflection data on BABEL Line 1

Marine normal-incidence reflection and P- and S-wave refraction/wide-angle reflection data from the BABEL project, line 1, in the Bothnian Sea have been used for investigating the nature of the reflectivity in the area. The data are available along the same line, and therefore the same piece of crust is probed by two seismic methods. We use this favourable circumstance for model calculations concerning both data sets by analysing the waveforms of reflections from normal-incidence traveltimes at approximately 4, 12 and 16 s TWT. Strong sub-horizontal reflections are observed on the normal-incidence reflection section at about 4 s TWT. These reflections have been modelled using the reflectivity method (1-D modelling) utilising the water bottom reflection as an estimate of the source wavelet and show a positive polarity. Thickness and P-wave velocity consistent with these reflections are 50 m and 7 km/s, respectively, with a host rock velocity of 6 km/s. From the polarity, velocity and thickness, these reflections are attributed to dolerite sills in the upper crust. Strong lower-crustal reflections are observed on the wide-angle and normal-incidence reflection sections. However, the corresponding S-wave reflections from the lower crust are considerably weaker. The implied high positive impedance contrast for the P-wave lower-crustal reflection and the weak S-wave reflectivity suggests the presence of a highly mafic sheet at about 40 km depth. The wide-angle P- and S-wave sections show strong reflections at the Moho. Coincident normal incidence data show only weak Moho reflectivity indicating a gradient in velocity at the crust/mantle boundary. Dipping events onlap the strong sub-horizontal lower crustal and the P-wave Moho reflections on the wide-angle data. These are explained by dipping structures above the lower crustal reflector and below the Moho, with the latter also consisting of a gradient in velocity.

Depolarization effect in a resonator with corner-cube reflectors

When an optical beam is incident into corner-cube reflector, it makes a large incidence angle with all three reflecting surfaces, and the polarization of the retroreflected beam depends greatly on the property of the reflecting surfaces. Knowledge of the polarization change is particularly important for a laser system with a polarization-dependent gain, such as a free-electron laser. By following the beam path in a corner cube reflector, one can study the depolarization effect by using 2 × 2 matrices. The results show that the change of polarization states is fairly sensitive to the phase difference between the s-wave and p-wave reflectivities of the surfaces. This sensitivity is greatly reduced if the two corner cubes in the resonator are oriented in the same direction.

The lithosphere below the Rockall Trough: wide-angle seismic evidence for extensive serpentinisation

The longer-range data from wide-angle seismic reflection experiments along an axial and transverse profile determined the seismic properties of the shallow mantle lithosphere beneath the Rockall Trough in the North Atlantic. Two subcrustal P-and S-wave reflections are observed. The first defines a layer 3–10 km thick below the Moho where P-wave velocities vary from 7.5 to 7.8 km/s. V p V s ratios increase within this layer from 1.80 in the north to 1.83 in the south of the basin along the axial profile. The second reflection from approximately 34 km depth identifies a layer 15–20 km thick, with a V p velocity of approximately 8.1 km/s and a V p V s ratio of 1.73. These values are typical for normal mantle peridotites. Both P- and S-wave velocities and V p V s ratios constrain the possible composition of the first layer which is interpreted as a zone of partially serpentinised peridotites below the Moho. About 15% volume alteration of the parent mantle peridotite is required to produce the observed seismic properties. This degree of alteration accounts for a systematic deficit in total tectonic subsidence when compared with that predicted from the variation in bulk crustal stretching along the axis of the basin. Syntectonic cooling occurred during differential lithospheric stretching, as the upper to mid-crust became more extended over a narrower region than the mantle lithosphere. This served to rheologically couple the lower crust to the mantle near the final stage of deformation as the primary brittle/ductile transition zone in the crust migrated downwards into the mantle lithosphere. The resultant fracturing generated the permeability necessary to facilitate the seawater circulation which hydrated the cold mantle.

Magma Distribution in Island Arc Mantle in Three Dimensions.

Distribution of partial melt (magma) in the upper mantle is investigated in detail from the seismic tomography of the mantle wedge beneath northeastern Japan. The comparison of the fine velocity structure with the laboratory velocity data on a partiall-ymolten mantle rock yields estimates of three-dimensional distribution of melt. The results indicate that the cause of island arc volcanism in northeastern Japan is the upwelling of hot mantle materials (volcanic plumes) from beneath. The source of volcanic magma is diapir-like melting regions localized within the volcanic plumes. Extensive volcanic activity at the volcanic front is due to the presence of the vast melting zones right beneath the Moho discontinuity. Those melting zones may cause melting of lower crustal materials and produce felsic magma. Melt stays underneath the Moho, where crystallization fractionation may proceed. Melt exists at greater depths in the back-arc region, which may cause across-arc variations of chemical compositions of the volcanic rocks in northeastern Japan. We suggest that magma migration in the ductile lower crust may cause low-frequency microearthquakes, and magma penetration into the brittle upper crust may produce mid-crustal S-wave reflectors.

S-wave anisotropy off Lofoten, Norway, indicative of fluids in the lower continental crust?

SUMMARY P- and S-wave velocity models along two 145 and 175 km long perpendicular profiles on the continental shelf off Lofoten, northern Norway, have been obtained from a study of nine three-component ocean-bottom seismographs. The S-wave model has been achieved from a study of the high-quality, horizontal component data. On one OBS the S-wave reflection from the Moho can be followed almost continuously from vertical to wide-angle incidence. This unique observation indicates the presence of about 10 per cent S-wave anisotropy in the lower crust along the NE-SW profile. No such anisotropy is observed along the NW-SE profile, and it is suggested that the inferred anisotropy might be caused by liquid-filled microcracks or pores aligned vertically along this profile. The vertical NW-SE alignment of the microcracks/pores might be a result of the present-day stress-field (the maximum compressive stress trends NW-SE), or it might be influenced by recent ductile strain fabrics or by ductile strain fabrics inherited from earlier deformation episodes. Another possible explanation for the inferred anisotropy might be alignment of anisotropic minerals. Many highly anisotropic minerals like olivine, pyroxene and hornblende can be excluded in this case, since the inferred S-wave anisotropy is at least three times higher than the P-wave anisotropy. Alignment of the mineral kyanite, however, might possibly explain the observations, since this mineral has much higher S-wave anisotropy than P-wave anisotropy. The hypothesis involving fluids is preferred since the very high seismic velocities of kyanite do not seem to be compatible with the estimated lower crustal velocity in this area (V, = 6.8 km s-').

Crustal “bright spots” and anisotropy from multi-component P- and S-wave measurements in southern Germany

Controlled shear-wave experiments have focussed on deep crustal targets that were earlier recognized by DEKORP P-wave surveys in southern Germany (Black Forest, Upper Palatinate). A prominent P-wave reflection (“bright spot”) at 8 km depth at the KTB deep drilling site is not seen on a comparable S-wave reflection section, in contrast to deeper reflections. This behaviour can be explained either by purely compositional effects (layer with anomalously high quartz content) or by fluid accumulation (“reservoir”) in a fracture zone. A 20 km deep, 10 km long prominent P-wave reflection in the southwestern Black Forest is only partially accompanied by corresponding S-wave reflections. The influence of the physical state of the rock e.g. stress-related cracking, in addition to possible compositional variations is suggested to explain the contrasting P- and S-wave reflection strength. Seismic anisotropy of 10% magnitude, revealed by S-wave splitting in vertical seismic profiling data at the KTB to more than 4 km depth, is attributed to sub-vertically dipping and NW-striking foliation.

Shear-wave splitting in the lower crust beneath the Archean crust of southwest Greenland

Slant stacks of seismic data from rifted ancient Archean crust along the southwest coast of Greenland indicate that the lower crust is strongly seismically anisotropic. The slant stack data show that a lower-crustal, wide-angle S-wave reflection has a different intercept time and ray parameter on the radial and transverse components from a receiver gather recorded on Passcal Reftek three-component seismometers. From the S-wave analysis, the continental crust is clearly seismically anisotropic above a high-velocity wedge in the lower crust. Possibly, magmatic underplating during Late Cretaceous Labrador Sea rifting heated the pre-existing lower crust promoting plastic flow and enabling alignment of anisotropic minerals to produce the seismic anisotropy.

High-resolution images of the lower crust: deep seismic reflections from 15 to 180 Hz

We report the results of an unconventional, broad-band deep-seismic reflection survey undertaken on land in southwest England. Previous marine seismic lines in this area have shown strong layered lower-crustal reflectivity. The broad-band survey shows near-normal-incidence P-wave reflections from the lower continental crust and from the Moho with frequencies that range from 15 to 180 Hz. High-frequency lower-crustal S-wave reflections were also observed. These high frequencies were obtained by placing both sources and receivers in boreholes up to 2 km deep, drilled within a large homogeneous granite pluton. Recording down-hole avoids loss of high frequency energy during transmission through heterogeneous and highly attenuative near-surface layers. The high frequencies were generated using explosive sources, and were recorded on sub-surface accelerometers and hydrophones, and on a conventional array of surface geophones. Seismic attenuation within the granite is low; the quality factor, Q, lies above 2000. These new, high-frequency observations show that the lower crust does not preferentially reflect particular frequencies. The data are most readily explained as resulting from the combination of a frequency-independent Q in the upper crust with a white reflectivity response in the lower crust. The data are not consistent with crustal models which are highly tuned to reflect preferentially the frequencies employed in conventional deep seismic reflection surveys. The data require relatively sharp boundaries within the lower crust; significant variation in physical properties must occur over vertical distances of not more than about 10 m. Sharp boundaries are most easily explained if the reflections result from lithological contrasts.

P- and S-wave reflections from anomalies in the lowermost mantle

Seismic P and S waves recorded at the GRF array in Germany are used to study the inhomogeneous structure of the boundary layer D″ at the base of the mantle. The use of the seismic array allows the detection of small-scale anomalies in the lowermost mantle. The lateral resolution attainable is about 100 to 400 km, i.e. more than 10 times better than with tomographic methods. The analysis of 13 years of GRF broad-band array data yields 255 events with high signal-to-noise ratio which are used to map the lowermost mantle. 74 of these events show anomalous P waves (PdP) which arrive 3–6 s after the direct P wave and have a slowness 0.7–0.8 s/° smaller than the slowness of direct P. Other events close to the events with PdP do not have such an anomalous phase. Using slowness, traveltime, amplitude and waveform information it is demonstrated that PdP is caused by an anomalous lower mantle velocity structure below the turning point of the P wave. If the pp phase (i.e. a P wave first reflected at the free surface near the source) is used together with the P phase, distinct and well-sepaarted P-velocity anomalies can be determined under the Nansen Basin, the Kara Sea and northern Siberia. The areas of the bounce points of PdP in the lower mantle have a lateral extension of about 100 by 200 km, but this is not the size of the anomaly, since the resolution of P waves at 1 Hz in this depth is 130 km by 260 km (Fresnel zone). The accuracy to which the depth of the reflector (2612 km under the Nansen Basin and 2605 km under northern Siberia respectively) can be determined for 1-D models is ±10 to 20 km. The velocity contrast at the lower mantle discontinuity is about 3 per cent ±1 to 1.5 per cent. Areas which have velocity fluctuations smaller than about 1 per cent can not be detected as anomalous areas. 2-D models of the anomalies reveal the range of adequate models and possible trade-offs. If the lateral extension of the anomaly is about 7° the reflector has to be 40 km deeper than in the 1-D model. If the dip of the reflector in the lowermost mantle is only about 1.5° it is difficult to resolve if the reflector is tilted towards the source or towards the receiver. For the anomaly under the Nansen Basin deviations from the great circle path are observed for PdP, indicating 3-D effects. Below the three anomalies the core-mantle boundary (CMB) can be located by PcP with a depth in agreement with standard earth models under northern Siberia has a region where P- and S-velocity anomalies coincide, but also a region where only an S-velocity anomaly but no P-velocity anomaly is observed. This results in changes of the Poisson ratio from +5.9 per cent to −4.8 per cent (±1.5 per cent) across the discontinuity in the lowermost mantle for the regions studied. The analysis of the S waves (S, SdS and ScS) reveals two S-velocity anomalies in the lowermost mantle under northern Siberia. The first anomaly coincides with the P velocity anomaly under northern Siberia and can be explained by a 1-D model with a reflector depth of 2610 km ± 15 km and a velocity contrast of 2.3 per cent. The second S-velocity anomaly is in an area where no P-velocity anomaly can be detected. The corresponding 1-D S-velocity model has a reflector depth of 2575 km ± 15 km and an S-velocity contrast of 2.6 per cent. The smallest structures that can be resolved with the S waves in this depth are about twice as large as for the P waves, i.e. 230 km by 460 km (Fresnel zone). The joint analysis of P and S waves therefore shows a region with a P-velocity anomaly together with weak indications for an S-velocity anomaly (Nansen Basin) and a second region with a P but no S anomaly (Kara Sea). The lowermost mantle under northern Siberia has a region where P- and S-velocity anomalies coincide, but also a region where only an S-velocity anomaly but no P-velocity anomaly is observed. This results in changes of the Poisson ratio from +5.9 per cent to −4.8 per cent (±1.5 per cent) across the discontinuity in the lowermost mantle for the regions studied.

Fluid reservoir (?) beneath the KTB drillbit indicated by seismic shear‐wave observations

Indications for the possible presence of a fluid accumulation below the drillbit of the KTB (German Continental Deep Drilling Project)‐Hauptbohrung are provided by comparisons of seismic P‐wave and S‐wave reflection sections. A bright reflection with negative polarity at approx. 8 km depth is seen only on P‐wave sections. It is absent on S‐wave sections in contrast to deeper reflections. This observation can be explained by the following geological models or combinations of them: (1) brine accumulation within a strongly fractured rock reservoir, (2) liquid/gas interface within strongly fractured rocks, (3) rock composition with increased quartz content and corresponding low Poisson's ratio, and (4) varying symmetry system of seismic anisotropy due to rock foliation. Predictions concerning the nature and location of this reflector are expected to be verified by drilling and direct probing in spring‐summer, 1993.

High Resolution Crosswell Imaging of a West Texas Carbonate Reservoir

Using crosswell data collected at a depth of about 3000 ft (900 m) in West Texas carbonates, one of the first well-to-well reflection images of an oil reservoir was produced. The P and S brute stack reflection images created after wavefield separation tied the sonic logs and exhibited a vertical resolution that was comparable to well log resolution. Both brute stacks demonstrated continuity of several reflectors known to be continuous from log control and also imaged an angular unconformity that was not detected in log correlations or in surface seismic profiling. The brute stacks, particularly the S-wave reflection image, also exhibited imaging artifacts. We found that multichannel wavefield separation filters that attenuated interfering wavemodes were a critical component in producing high-resolution reflection images. In this study, the most important elements for an effective wavefield separation were the time-alignment of seismic arrivals prior to filter application and the implementation of wavefield-separation filters in multiple domains, particularly in common offset domain. The effectiveness of the multichannel filtering was enhanced through the use of extremely fine wellbore sampling intervals. In this study, 2.5 ft (0.76 m) vertical sampling intervals for both source and receiver were used, whereas most previous crosswell data sets were collected with much coarser sampling intervals, resulting in spatial aliasing and limiting the utility of the data for reflection processing. The wavefield separation techniques employed in this study used data volumes and associated filtering operations that were several orders of magnitude larger than those encountered in conventional VSP data analysis.

Mapping of quaternary deposits and bedrock topography using high-resolution seismic reflection techniques

Marine normal-incidence reflection and P- and S-wave refraction/wide-angle reflection data from the BABEL project, line 1, in the Bothnian Sea have been used for investigating the nature of the reflectivity in the area. The data are available along the same line, and therefore the same piece of crust is probed by two seismic methods. We use this favourable circumstance for model calculations concerning both data sets by analysing the waveforms of reflections from normal-incidence traveltimes at approximately 4, 12 and 16 s TWT. Strong sub-horizontal reflections are observed on the normal-incidence reflection section at about 4 s TWT. These reflections have been modelled using the reflectivity method (1-D modelling) utilising the water bottom reflection as an estimate of the source wavelet and show a positive polarity. Thickness and P-wave velocity consistent with these reflections are 50 m and 7 km/s, respectively, with a host rock velocity of 6 km/s. From the polarity, velocity and thickness, these reflections are attributed to dolerite sills in the upper crust. Strong lower-crustal reflections are observed on the wide-angle and normal-incidence reflection sections. However, the corresponding S-wave reflections from the lower crust are considerably weaker. The implied high positive impedance contrast for the P-wave lower-crustal reflection and the weak S-wave reflectivity suggests the presence of a highly mafic sheet at about 40 km depth. The wide-angle P- and S-wave sections show strong reflections at the Moho. Coincident normal incidence data show only weak Moho reflectivity indicating a gradient in velocity at the crust/mantle boundary. Dipping events onlap the strong sub-horizontal lower crustal and the P-wave Moho reflections on the wide-angle data. These are explained by dipping structures above the lower crustal reflector and below the Moho, with the latter also consisting of a gradient in velocity.

Porosity interpretation through seismics : Geoexploration V28, N1, April 1991, P91–106

Abstract This paper describes a procedure for porosity interpretation from seismic data (POISE) by exploiting merits of several recent advances in seismic data processing and interpretation. Briefly, the procedure is as follows: starting from NMO corrected CDP gathers of P-wave data and applying amplitude versus offset (AVO) analysis, one can separate out zero offset “P-wave” and “Swave” stacks both as a function of P-wave time. Here the term “S-wave” refers to a stack that we would have obtained if we had carried out an S-wave survey and displayed the data in P-wave traveltime. Application of maximum likelihood deconvolution/modelling technique on stacks leads to models for P-wave and S-wave reflection coefficients. Finally, inverting the reflection coefficients for interval transit time, a section can be prepared to display ΔT s - ΔT p , the difference in slowness of the S-wave and P-wave as a function of the P-wave traveltime. The final output of POISE for ΔT s - ΔT p is particularly useful for studying variations in lithology and porosity within a formation. The interpretation is fouSnded on an empirical observation by Ikwuakor that ΔT s - ΔT p from character logs for a large number of carbonate and sandstone samples, is proportional to the core porosity (for a given rock type). The utility of POISE is illustrated with a field example in a carbonate reservoir.

Deep structure of the northeastern Japan arc and its relationship to seismic and volcanic activity

Evidence for possible deep-seated magmatic activity beneath northeastern Japan can be obtained from seismic observations. Tomographic inversions of P-wave velocity data show low-velocity zones distributed in the crust and upper mantle beneath active volcanoes. In or around these zones, anomalously low-frequency micro-earthquakes, perhaps caused by magmatic activity, are found at depths of 25–40 km; distinct S-wave reflectors, corresponding to the upper surfaces of magma bodies, are found at depths of 10–18 km. Large crustal earthquakes also occur around the low-velocity zones. These observations reveal the distribution of magma reservoirs at depths and elucidate its relation with shallow seismicity beneath volcanic arcs.

Porosity interpretation through seismics

This paper describes a procedure for porosity interpretation from seismic data (POISE) by exploiting merits of several recent advances in seismic data processing and interpretation. Briefly, the procedure is as follows: starting from NMO corrected CDP gathers of P-wave data and applying amplitude versus offset (AVO) analysis, one can separate out zero offset “P-wave” and “Swave” stacks both as a function of P-wave time. Here the term “S-wave” refers to a stack that we would have obtained if we had carried out an S-wave survey and displayed the data in P-wave traveltime. Application of maximum likelihood deconvolution/modelling technique on stacks leads to models for P-wave and S-wave reflection coefficients. Finally, inverting the reflection coefficients for interval transit time, a section can be prepared to display ΔT s - ΔT p , the difference in slowness of the S-wave and P-wave as a function of the P-wave traveltime. The final output of POISE for ΔT s - ΔT p is particularly useful for studying variations in lithology and porosity within a formation. The interpretation is fouSnded on an empirical observation by Ikwuakor that ΔT s - ΔT p from character logs for a large number of carbonate and sandstone samples, is proportional to the core porosity (for a given rock type). The utility of POISE is illustrated with a field example in a carbonate reservoir.

Static corrections from shallow‐reflection surveys

Shallow seismic reflection surveys can assist in determination of velocity and/or thickness variations in near‐surface layers. Static corrections to seismic reflection data compensate for velocity and thickness variations within the “weathered zone.” An uncompensated weathered‐layer thickness variation on the order of 1 m across the length of a geophone array can distort the spectrum of the signal and result in aberrations on final stacked data. P-wave velocities in areas where the weathered zone is composed of unconsolidated materials can be substantially less than the velocity of sound in air. Weathered‐layer thickness variation of 1 m in these low‐velocity materials could result in a static anomaly in excess of 3 ms. Shallow‐reflection data from the Texas panhandle illustrate a real geologic situation with sufficient variability in the near surface to significantly affect seismic signal reflected from depths commonly targeted by conventional reflection surveys. Synthetic data approximating a conventional reflection survey combined with a weathered‐layer model generated from shallow‐reflection data show the possible dramatic static effects of alluvium. Shallow high‐resolution reflection surveys can be used both to determine the severity of intra‐array statics and to assist in the design of a filter to remove much of the distortion such statics cause on deeper reflection data. The static effects of unconsolidated materials can be even more dramatic on S-wave reflection surveys than on comparable P-wave surveys.

Shear-wave evidence for an anisotropic lower crust beneath the Black Forest, southwest Germany

Controlled shear-wave sources (a horizontal vibrator and dynamite) were used in near-vertical reflection studies to explore the nature of the reflective laminated lower crust in southwest Germany. The data reveal strong S-wave reflections from the lower crust. These lower crustal S-reflections were not observed by previous wide-angle surveys in the same region. A laminated crustal model with isotropic layers (and changing Poisson's ratio) cannot explain the differing near- and far-offset S-response. By introducing alternating anisotropic and isotropic lamellae, this discrepancy may be resolved. This Seismic model is consistent with a petrological model of deformed amphibolites containing 10–30% of preferentially orientated hornblende. Observed amplitude differences in P- and S-wave reflections from the same depth in the lower crust suggest regions with a changing Poisson's ratio.